The 120 mortar is a highly used strong point for the Soviets for mid-game.
Replacing it will just make another boring faction, because EVERY faction has THE SAME mortar type damage. (All VERY Close, I should say)
None of them are as powerful as the Soviets.
However, the Soviets lack good infantry in start-game :P
It's kinda what makes them unique, their support center.
You'll find if you spam Tank Hunters and use Red Tide, MOST German tanks CAN'T stop them.
I was in a 4 v 4 in Berlin, with 1 teamate who quit early. (So 3 CPUS) VS 2 of my friend on teamspeak, with 2 CPUS.
One of my friends went PE, and he was SPAMMING panthers like no end, and had a JagdPanther.
Tanks of mine couldn't even get CLOSE to him. Even IS-2s were obliterated.
So the thought popped in my head as his tanks approached me, close, close, close.
"TANK HUNTERS FTW?"
So, with the upgrade, I had about 8 squads of PRTD Tank Hunters.
Surprisingly, they took out a Half-Health Jagdpanther, THREE Panthers, a Hetzer, and a Bergetiger before dying.
My friend nearly pissed himself when he saw that my squads were not only all back in good shape after he killed them with his last 2 panthers, but I had also sent in 2 more PRTD and 2 Red Banner Strelky to counter Panzer Grenadiers.
They obliterated the panthers and the RBS PPSH-41 tore the Panzer Grenadiers to shreds.
It's all about combination of good units IMO. The Mortar just adds a good mid game, not so useful late game.
"Balancers are 10 a penny"